1. Field of the Invention
The invention relates to a fuel cell system and an operating method of a fuel cell which prevent the uneven distribution of water on the surface of a single cell.
2. Description of the Related Art
Fuel cells convert chemical energy directly into electric energy by supplying a fuel and an oxidant to two electrodes that are electrically connected together, and electrochemically oxidizing the fuel. Unlike thermal power generation, fuel cells are highly efficient in converting energy because they are not limited by the Carnot cycle. Fuel cells are normally formed of a stack of a plurality of single cells, each of which is basically made up of a membrane electrode assembly (MEA) in which an electrolyte membrane is sandwiched between a pair of electrodes. Among fuel cells, polymer electrolyte fuel cells having a polymer electrolyte membrane as the electrolyte membrane are particularly attractive as portable power supplies and power supplies for movable objects because they can easily be made small and operate at low temperatures.
In polymer electrolyte fuel cells, when hydrogen is used as the fuel, the reaction in Expression (1) below takes place at the anode (i.e., the fuel electrode).H2→2H++2e−  (1)The electrons that are freed as a result of Expression (1) above pass through an external circuit where they perform work at an external load and then reach the cathode (i.e., the oxidant pole). There, the protons created by Expression (1) move through the polymer electrolyte membrane from the anode to the cathode in a hydrated state from electro-osmosis.
Also, when oxygen is used as the oxidant, the reaction in Expression (2) takes place at the cathode.2H++(½)O2+2e−→H2O  (2)The water produced at the cathode passes mainly through a gas diffusion layer, after which it is discharged out of the fuel cell. In this way, the fuel cell is a clean power source that emits nothing but water.
If there is an excessive amount of water (in this specification, the term “water” is used in a broad sense and thus also includes moisture and the like) that is to be emitted, water that has condensed in the fuel cell may block gaps in the catalyst layer or the gas diffusion layer, and in an extreme case, even block the gas flow path and thus impede the supply of gas, preventing a sufficient amount of gas for generating power from reaching the catalyst layer, which would cause the output of the fuel cell to decrease. Also, if there is not enough water in the fuel cell, it will increase the internal resistance, also causing the output and power generating efficiency of the fuel cell to decrease. Various technologies, which are described below, have previously been developed to solve this problem.
Japanese Patent Application Publication No. 2002-352827 (JP-A-2002-352827) describes technology relating to a fuel cell system that includes i) amount of water determining means for determining an excess or insufficiency in the amount of water in the fuel cell, and ii) gas supply amount controlling means for controlling the gas supply amount of oxygen and/or hydrogen supplied to the fuel cell, based on the determination results from the amount of water determining means regarding an excess or insufficiency in the amount of water in the fuel cell.
Japanese Patent Application Publication No. 2006-210004 (JP-A-2006-210004) describes technology relating to a fuel cell system that includes i) air amount adjusting means for adjusting the amount of air flowing in an oxidant gas flow path, ii) determining means for determining whether an electrolyte membrane is drying out, and iii) controlling means for controlling the pressure of air flowing in the oxidant gas flow path so that it becomes higher than it is during normal operation, when it is determined that the electrolyte membrane is drying out.
Published Japanese Translation of PCT application No. 6-504403 (JP-A-6-504403) describes technology for removing water in a cathode by adjusting the water vapor partial pressure in the fuel gas so that water or water vapor moves from the cathode to the anode through the electrolyte membrane.
Japanese Patent No. 3736475 describes technology for promoting the supply of amount of water to a fuel gas flow path inlet portion via an electrolyte membrane by flocculating and recovering the water in the oxidant gas. This is achieved by providing a fuel gas flow path and an oxidant gas flow path such that fuel gas and oxidant gas flow opposing each other in the fuel cell, and further providing a coolant path for cooling the oxidant gas flow path outlet portion.
Japanese Patent Application Publication No. 2001-6698 (JP-A-2001-6698) describes technology for suppressing the evaporation of water vapor into oxidant gas by reducing the drying rate in an oxidant gas flow path. This is achieved by making the temperature at the oxidant gas flow path inlet lower than the temperature at the oxidant gas flow path outlet when the fuel cell is operating, and making the gas diffusion of the diffusion layer at the oxidant gas flow path inlet lower than the gas diffusion at the oxidant gas flow path outlet.
Water tends to be unevenly distributed on a surface of a single cell of a fuel cell supplied with non-humidified fuel gas and oxidant gas is operated, i.e., during non-humidified operation. That is, the area near the oxidant gas flow path inlet tends become dry, while the area near the oxidant gas flow path outlet tends to become wet. Therefore, it is necessary to try to homogenize the amount of water on the surface of the single cell. This is also true when the temperature of the membrane electrode assembly is high, i.e., 70° C. or higher. The technologies described in JP-A-2002-352827 and JP-A-2006-210004 both include means for solving the problem, i.e., they both determine excess or insufficiency in the amount of water of the overall fuel cell and adjust the gas based on that determination. However, neither of these technologies considers the uneven distribution of water on the surface of a single cell. Therefore, neither of these technologies is considered to be effective when there is too much or too little water on the surface of a single cell. In particular, in the controlling means described in JP-A-2006-210004, the means for solving the problem by increasing the air pressure is thought to have the drawback of reducing fuel efficiency due to the increase in air compressor output. The technologies described in JP-A-6-504403, Japanese Patent No. 3736475, and JP-A-2001-6698 all have means for solving the problem that involve passing the water accumulated near the mainly the outlet of the oxidant gas flow path through the electrolyte membrane to the fuel gas flow path. In particular, the technologies described in Japanese Patent No. 3736475 and JP-A-2001-6698 have means for solving the problem that involve circulating water from near the oxidant gas flow path outlet→near the fuel gas flow path inlet→near the fuel gas flow path outlet→near the oxidant gas flow path inlet, because the oxidant gas flow path and the fuel gas flow path are opposing. However, even with the technologies described in JP-A-6-504403, Japanese Patent No. 3736475, and JP-A-2001-6698, determining means for determining whether there is an excess or insufficiency in the amount of water, as well as gas controlling means for adjusting the gas based on the determination made by that determining means, are not taken into consideration. Also, the technology described in JP-A-6-504403 is intended to prevent flooding in the oxidant gas flow path, not to prevent the area near the oxidant gas flow path inlet from drying out.